This project deals with the physical factors which affect an exchange of fluid between peripheral blood vessels, the interstitial space, and the related intracellular space. Because immersion of the body in warm water results in a transfer of fluid into the blood, a determination has been made of the plasma electrolye composition and of the interstitial fluid pressure and composition during immersion diuresis, and also after the first few minutes of compensatory fluid shifts, during hemorrhagic hypotension. A vapor pressure osmometer has been used for osmotic determinations after water loss from whole blood and red blood cells. The exchange of electrolytes, amino acids, and bicarbonate between the intracellular space and intravascular space was measured in order to reconcile red cell volume changes, with the osmotic pressure changes which have been found in our recent immersion experiments on man. A study of hydrostatic forces reveals transmural pressure distributions in the bodies of vertebrates due to gravity. Compensatory mechanisms have evolved to redistribute blood flow and fluid volume to overcome the changes in these hydrostatic pressure gradients due to changes in posture, space flight, or immersion. The mechanism of vascular fluid volume to overcome the changes in these hydrostatic pressure gradients due to changes in posture, space flight, or immersion. The mechanism of vascular fluid volume redistribution in the face of such pressures is being explored in aquatic and terrestrial vertebrates. Pressure inside the blood vessels and pressure outside the skin (in water) are being investigated because vascular transmural filtration pressures are affected by both of these. The effect of skin temperature on these filtration pressures and on interstitial fluid pressure will be measured on humans and animals.